The xylenes, such as para-xylene, meta-xylene and ortho-xylene, can be important intermediates that find wide and varied application in chemical syntheses. Generally, para-xylene upon oxidation yields terephthalic acid that is used in the manufacture of synthetic textile fibers and resins. Meta-xylene can be used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Generally, ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which can be difficult to separate or to convert. Typically, para-xylene is a major chemical intermediate with significant demand, but amounts to only 20-25% of a typical C8 aromatic stream. Adjustment of an isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Typically, isomerization converts a non-equilibrium mixture of the xylene isomers that is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization. In selecting an appropriate technology, it is desirable to run the isomerization process as close to equilibrium as practical in order to maximize the para-xylene yield; however, associated with this is a greater cyclic C8 loss due to side reactions. Often, the approach to equilibrium that is used is an optimized compromise between high C8 cyclic loss at high conversion (i.e., very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C8 aromatic. Thus, catalysts can be evaluated on the basis of a favorable balance of activity, selectivity and stability.
Catalysts can be made by several different processes. Generally, producing an extruded catalyst that can isomerize ethylbenzene to xylenes while minimizing C8 ring loss would be beneficial.